Project Summary/Abstract The striatum is the main input structure of the basal ganglia, a system that is crucial not only for voluntary motor control, but also for reinforcement-mediated learning and higher cognitive functions. The importance of the striatum is illustrated by the severe disabilities associated with numerous neurological and neuropsy- chiatric conditions that affect this brain structure. The recent introduction of methods for targeting and manipulating genetically and physiologically defined cell types is currently revolutionizing our understanding of the neostriatum. Our preliminary studies, together with recent research demonstrate that the classically known cell types represent less than a third of the interneuron classes in the striatum, and reveal an intricate and precisely organized circuitry of these neurons. The prosed study will test 4 novel hypotheses which were formulated on the basis of preliminary data and capture functionally important principles of organization of the network of striatal interneurons. First, we will investigate the connectivity of genetically identified interneurons and test the hypotheses that their connectivity is highly cell type specific and structured to form at least 2 separate interneuron sub- networks within the neostriatum. Second, building on our previous research and novel preliminary data we hypothesize that cholinergic interneurons (CINs), which traditionally have been thought of as neuromodulatory elements, participate in a fast bi-directional synaptic network with multiple GABAergic interneurons utilizing nicotinic excitatory connections. Importantly, our preliminary data also suggest the existence of a novel source of nicotinic excitation from the brainstem that appears to selectively target a distinct subset of GABAergic interneurons. Third, we will test the hypothesis that a subset of interneurons are hierarchically organized in the sense that one or more classes of interneurons exist which are specialized to control the activity of other interneurons. These interneuron-specific interneurons are of great interest because their impact may be widely distributed and disproportionally amplified via hierarchical control of other interneurons. Finally, based on preliminary data we hypothesize that a class of novel GABAergic interneurons are essential for normal goal-directed behavior, and we will explore the mechanism of action of these neurons using in vivo electrophysiological and Ca2+-imaging methods. These experiments will yield important new insights into the functioning of the striatum and may help to identify new cellular substrates for therapeutic interventions in a variety of neurological and neuropsychiatric disorders.